"What you're looking at is the largest welding system in the world," said Jackie Nesselroad. She is leading a team from Boeing that's welding together the world's most powerful rocket, the Space Launch System. "It's about the coolest job on earth."

It is designed for beyond low-Earth orbit exploration with humans," said Frank McCall, the deputy program manager from Boeing who called SLS "a mission that is long overdue." The first unmanned test flight is slated for late 2018. By 2021, the rocket is supposed to carry astronauts aboard the Orion space capsule built by Lockheed Martin.

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The SLS program is hoping for continued funding to pay for more trips, because beyond the first two missions, the way to Mars isn't clear. Perhaps the rocket system could take astronauts back to the moon, or to an asteroid. It all depends on how the first two missions go, how much it costs, and what other competing priorities there are through 2035.